1. Field of the Invention
The present invention relates to a computer implemented method for development profile simulation, a computer program product for controlling a computer system so as to simulate development profile, and a computer implemented method for mask pattern data correction and in particular to a simulation method to predict a resist pattern shape after exposure and development and a computer program for the simulation method and a mask pattern data correction method using the predicted pattern shape.
2. Description of the Related Art
Generally, a series of processes for manufacturing semiconductor devices includes a lithography process to make a fine pattern on a substrate. In the lithography process, various parameters such as exposure conditions for a projection system and resist process conditions are present to provide a desired resist pattern. For example, the exposure conditions for the projection system include an aperture, a coherence factor, the shape of a light source and a pupil filter. The resist process conditions include the types and thicknesses of resist and anti-reflection films, a post-exposure baking (PEB) temperature, a PEB time, and development time. Furthermore, the parameters for the lithography process also include reticle conditions such as mask pattern size and mask type to provide the desired resist pattern.
As semiconductor devices have continued to shrink in size over time, it is impossible to obtain the desired resist pattern with a desired depth of focus without optimizing the parameters described above, because of a narrow margin of the depth of focus. However, optimizing the parameters for the lithography process by actually manufacturing masks over and over requires a great deal of labor, time and costs. Therefore, in order to reduce the labor, time and costs, a computer simulation is employed to estimate the shape of a developed photosensitive resist. Various resist profile simulation models such as a string model, a ray tracing model and a cell model having been proposed. For the existing resist profile simulation models, it is assumed that the dissolution rate of the photosensitive resist with the developer depend only on the exposure dose, the photosensitive character of the resist and the process condition, or only on the resist pattern shape at a dissolved portion in addition to the above described items.
However, the dissolution rate of the photosensitive resist during the actual development is affected by the change in alkaline concentration. For example, since the resist is developed by the neutralization reaction between an acid unit in the resist and the alkaline in the developer, the alkaline concentration of the developer varies as the neutralization reaction progresses. Therefore, it is difficult to assume that the existing resist profile simulation models accurately predict the resist pattern shape and the resist pattern size.
In Japanese Patent Laid-Open Publication No. Hei 10-64792, a resist profile simulation model that takes into account the change in the alkaline concentration is proposed. According to the proposed resist profile simulation model, the dissolution rate is adjusted in accordance with either the spatial average value of a latent image intensity or a photosensitizer concentration on the assumption that the change in the alkaline concentration is proportional to the amount of dissolved resist around a target resist pattern. The proposed resist profile simulation model assumes that the changing ratio of the dissolution rate is proportional to the amount of the dissolved resist around the target resist pattern, i.e., the change in the alkaline concentration.
However, when the dissolution rate is calculated by an actual measurement, the changing ratio of the dissolution rate is not always proportional to the change in the alkaline concentration. Therefore, the existing resist profile simulation models may fail to converge on a calculation to optimize the parameters. Even when the parameters are optimized, the predicted resist pattern size obtained by the simulation may not be equal to the actual resist pattern size obtained by actual measurement.
Further, when a developer is supplied to the resist, the developer flows in a direction in accordance with the flow velocity. In the case of a scan development method where a developer nozzle is scanned a wafer from one end to the other, the flow of the developer may occur during the development, after a puddle is formed. Since the alkaline concentration of the developer is reduced by the dissolution of the resist, the concentration of the developer varies on the concentration of the resist pattern. Therefore, the actual resist pattern size may differ from the predicted resist pattern size. Consequently, the existing resist profile simulation models also fail to predict the resist pattern size variation due to the flow of the developer.